I don't know if you're meeting any italians, but if you do, and you need to signal 5, never use all fingers with the palm outwards. its like the v-sign discussed above.
Uh, I am an Italian and I never knew that — actually, that's precisely the normal way in which I have always seen people signaling 5. Perhaps that's a matter of regional differences?
I have been in England, and I think that you should not have any problem there — you will
be recognized as a tourist, and any minor faux pas you will end making will be understood and forgiven (as long as you don't go out of your way to be obnoxious, of course).
The only thing I found a little bizarre about England was that waitresses kept calling me "sweetie". It sounded oddly personal to me, but eh, no harm done.